Corrugated transition device for use between a continuous and a corrugated circular waveguide with signal in two different frequency bands

ABSTRACT

A transition device achieves transformation of the signal carrier mode of a continuous wave guide, into the hybrid mode, the corresponding mode for carrying signals in corrugated structures, by employing a tapered waveguide transition of circular cross-section having dual-depth circumferential slots in the interior boundary surface thereof. The transition device utilizes a mutual resonance property of the slots at the port which connects to a continuous waveguide to achieve satisfactory operation in two frequency bands. At the port which is connected to a corrugated horn, the quarter wavelength self resonance of the individual slots provides the desired hybrid mode under balanced hybrid condition in these two bands. A gradual transition of the electrical characteristics is achieved along the length of the transition device through an adjustment of slot dimensions. Excitation of higher order spurious modes is maintained at a low level when properly chosen cross-sectional dimensions are considered along the length of the transition device.

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates to a device for propagating signals between acontinuous and a corrugated circular waveguide with minimized mismatchand low spurious mode excitations in two bands of frequency realizedthrough a special inner boundary configuration in the transition whichconsists of dual-depth corrugations with changing dimensions along thelength thereof.

II. Background Information

It is well known, satellite communication systems operate through theuse of two distinct and well defined frequency bands where the higherfrequency band (uplink) carries signals from the earth stations to thesatellite while signals are sent from the satellite towards the earthstations in the lower frequency band (downlink). For such applicationswith certain stringent electrical specifications imposed on theradiation characteristics of the operating antennas, a corrugated hornfeeding the reflector antenna system is considered to be one of theoptimum solutions. This arrangement achieves satisfactory efficiencywhile maintaining low sidelobe and cross-polarized radiation levels.

With the introduction of the concept of frequency reuse where betterutilization of the available frequency bands through simultaneouspropagation of signals via two orthogonal polarizations at the samefrequency is considered, the electrical specifications on the antennacharacteristics have become furthermore stringent. In order to fulfilthese requirements in terms of the cross-polarized radiationcharacteristics, often a dual-depth corrugated horn is employed whichallows very low cross-polarized radiation characteristics to bemaintained in two widely separated frequency bands, with an availablefreedom for adjustment of separation between the two bands.

However, for both the above mentioned applications utilizing a horn withconventional or dual-depth corrugations, the horn is conventionallyconnected at its throat region to a continuous circular waveguide whichconstitutes the common transmission line of the feed chain for theuplink as well as the downlink signals. The continuous circularwaveguide supports the signals as the dominant TE11 mode. Thearrangement calls for a transition to be devised to transform this modeinto HE11 hybrid mode that propagates along the corrugated configurationof the horn. There are certain deleterious effects such as high returnloss of the signals or unacceptable levels of spurious mode excitationthat may accompany the transformation of TE11 to HE11 mode in thetransition from a continuous circular waveguide to a corrugated circularwaveguide, especially, when such transformation is desired at two widelyseparated frequency bands simultaneously.

In order that such a transition functions satisfactorily, a highsusceptance boundary condition must be simulated near the continuouswaveguide end through usage of appropriately configured corrugationswhich must gradually change their dimensions along the length of thetransition to reach a low susceptance boundary condition at the otherend where it connects into the horn. The manner of changing thecorrugation configuration along the length of transition together withchange in cross-section of the transition, is based on certain designcriterion which prevents excitation of spurious modes or introduction ofreturn loss at unacceptable levels.

Amongst the known transition for the transformation of TE11 to HE11modes, there are two principal types which present satisfactory resultsfor many applications. The first and most commonly used type oftransition consists of a conventionally corrugated tapered circularwaveguide transition where the depth of the corrugations are about halfa free space wavelength deep at the highest frequency of operation atthe continuous waveguide end, and starting with this value of depth ofcorrugations, they are diminished in depth gradually along the length ofthe transition such that about a quarter of a wavelength deep slot atthe lowest frequency of operation is achieved at the end connecting intothe horn. Such a transition operates with satisfactory electricalcharacteristics over a single and reasonably broad band. However, such atransition fails to operate satisfactorily when optimized performance isdesired in two widely separated bands. The second and the ratherinvolved, in terms of its manufacturing, type of the transition consistsof a tapered circular waveguide transition furnished with a specialcorrugated boundary made of ring loaded corrugations. These ring loadedcorrugations have a wider opening at the bottom to achieve broadenedband of operation that encompasses the widely separated bands.

In terms of manufacturing, due to the unusual shape of the corrugations,the ring loaded corrugation configuration presents many difficulties.Since conventional machining techniques cannot be used to make suchcorrugations, they must be either configured with discs or electroformedon a mandrel which is later removed by chemical dissolving. Needless toemphasize, such methods of manufacturing call for considerable amount ofeffort and cost in production. Of course, in terms of the electricalperformance, this second type of transition can potentially achieve thedesired specification far more satisfactorily than the first typediscussed before.

SUMMARY OF THE INVENTION

With the above described background on the state of the art on thedesign of the transitions between continuous and corrugated circularwaveguides which operate in two separated frequency bands, the objectiveof this invention has, therefore, been to develop an efficient dual-bandtransition between a continuous and a corrugated circular waveguidewhich is, at the same time, a sufficiently simple configuration that canbe manufactured by conventional machining techniques.

The present invention is a transition in circular cross-section with itsinner boundary wall furnished with circumferential dual-depthcorrugations which allow efficient transformation of TE11 mode of acontinuous circular waveguide into HE11 mode of a corrugated circularwaveguide for two widely separated bands of frequencies. Hereafter theinvention will be referred to as "dual-depth corrugated transition" orsimply DDCT. The corrugations in the DDCT are formed by a plurality ofcircumferential slots which are classified into two distinct types interms of the differences in the relative depth and sometimes also thewidth of the slots. These two types of slots are interspread betweenthemselves so that in the resulting corrugated configuration, successiveslots are of different types while alternate slots are of a common type.At that end of the DDCT which connects into the horn, the two types ofslots are optimized in their depths in such a way that each one of themis in quarter wavelength self resonance at different frequencies whichare assigned to belong, one each, to the two separated bands ofinterest. As a result of this, each self resonant slot presents a lowsusceptance in the band where its resonant frequency is located whilethe adjacent non-resonant slot contributes very little towardsdeterminning the net susceptance boundary condition. Hence, a net lowsusceptance boundary condition is suitably simulated in two bandssimultaneously to support HE11 mode at that end of the DDCT whichconnects to the horn. Whereas, at the end of the DDCT connnecting withthe continuous waveguide, the two types of slots are given certainamount of increased depths such that at the two pre-assigned frequencieswhich belong to the two bands of interest, the adjacent slots of twodistinct types are in mutual resonance to give a resultant highsusceptance boundary condition in the two bands simultaneously. Themutual resonance between the adjacent slots is caused by placement oftheir individual susceptances in such a way that they are comparable inmagnitude but opposite in sign, i.e, one is capacitive and the other isinductive. In this way, the desired high susceptance boundary conditionis simulated in the continuous waveguide end of the DDCT to achievesatisfactory matching condition for the TE11 mode at two frequency bandssimultaneously. Finally, along the length of the DDCT a gradual changein dimension, predominantly the depth and sometimes also the slotwidthand corrugation wall thickness, for both types of corrugation slots isconsidered to incorporate a gradual change of boundary condition betweenthe two ends.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in and further described with reference tothe accompanying FIGS. 1 to 3 in which:

FIG. 1 shows a cross-sectional view of the DDCT consisting of dual-depthcorrugations with changing depth of slots along the length of thestructure.

FIG. 2 shows the susceptance of the individual corrugation slots, whichconstitute the dual-depth corrugations, and the resultant simulatedsusceptance at the downlink along the length of the DDCT.

FIG. 3 shows the susceptance of the individual corrugation slots, whichconstitute the dual-depth corrugations, and the resultant simulatedsusceptance at the uplink along the length of the DDCT.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Refering to the FIG. 1, the DDCT consists of a metal body 10 which hasan internal circular cross-section surface with a plurality ofcorrugation forming slots, 14 and 15. The annular irises 16 separate theslots, 14 and 15, to create a corrugation boundary of the DDCT in whichthe slots are classified into two types: one series of slots, referenced14, have greater depth and a certain width while the second series ofslots, referenced 15, have a relatively smaller depth and optionally adifferent width also. The plurality of the above mentioned two types ofslots are alternately positioned to give rise to a dual-depthcorrugation boundary where the successive slots are of the differenttype, i.e, 14 and 15; while the alternate slots are of a common type,i.e., 14 and 14 or 15 and 15. Furthermore, along the length of the DDCTbetween the ports 12 and 13, the dual-depth corrugation boundaryundergoes a continuous dimensional change, predominantly, in terms ofthe depth of slots; although, in some cases, the change may also includevariation in the width of slots or the width of irises. The port 12 ofthe DDCT is connected to a continuous circular waveguide 11; whereas,port 13 is connected to the throat of a horn (not shown in figure).

In order to explain the functioning of the DDCT, shown in FIG. 1,reference will be made to FIGS. 2 and 3 which show the susceptances(17,18) and (25,26) of the individual slots 14 and 15, constituting thedual-depth corrugations and the resultant simulated susceptances (19 and27) along the length of the DDCT for the downlink and uplink,respectively. A high susceptance corrugation boundary condition isanalogous to the natural boundary condition of a continuous waveguideand, therefore, the corrugations near the port 12 in the DDCT should beso configured that a high resultant susceptance boundary condition issimulated for both the links. This boundary condition is simulated inthe present invention by means of an induced mutual resonance betweenthe adjacent slots of different type in the dual-depth configurationnear the port 12. The mutual resonance between the adjacent slots isachieved by the placement of susceptances of individual adjacent slotsat comparable non zero magnitude but associated with oppositecharacteristics such as capacitive and inductive susceptances. Forexample, at the downlink, the deep slots 14 present a capacitive (+ve)susceptance 20 while the shallow slots 15 present an inductive (-ve)susceptance 21 near the port 12; as a consequence of which, the twosusceptances combine and give rise to a mutual resonance to simulate thehigh susceptance 23. Next, in case of the uplink, the deep slots 14present an inductive (-ve) susceptance 28 and the shallow slots 15present a capacitive (+ve) susceptance 29 which mutually resonate togive, once again, the resultant high susceptance 31 at the port 12. Awayfrom the port 12 as the opposite end, port 13, of the DDCT isapproached, the corrugation boundary must be able to simulate a nearlyzero susceptance in order to support the HE11 hybrid mode near balancedhybrid condition, which is the wanted mode for propagation in thecorrugated horn. This susceptance boundary condition near the port 13 isconceived by an optimized depth of the slots in the dual-depthconfiguration so that a quarter wavelength self resonance for theindividual slots of the two types is achieved at two differentfrequencies which are located, one each, in the two links underconsideration. Specifically, for the example considered in FIGS. 1, 2and 3, the depth of the slots 14 furnishes self resonant low susceptancecondition 22 in the downlink and the optimized depth of the slots 15provides self resonant low susceptance condition 30 in the uplink. Nearthe self resonant condition of a slot in a particular frequency band,the susceptance of the adjacent slot, which is under non-resonantcondition, has less influence in determining the resultant susceptanceof the corrugation boundary. Hence, near the port 13, the simulatedboundary susceptances 24 and 32 for the downlink and uplink,respectively, are predominantly decided by the susceptances 22 and 30which represent operation near quarter wavelength resonant condition forthe slots 14 and 15, respectively. Along the length of the DDCT agradual change in the configuration of the slots is achieved to allowfor a continuous transition from the high susceptance boundary conditionat port 12 to low susceptance boundary condition at port 13. In FIG. 2,the susceptances 17, 18 and 19 show the variation in the downlink forthe individual slots 14, 15 and the resultant of the two combined,respectively. In FIG. 3, similarly, the susceptances 25, 26 and 27 showthe variation in the uplink for the corresponding cases.

It is important to note from what has been described above that asatisfactory match can be achieved in a transition between a continuousand a corrugated circular waveguide by utilizing the principles of theabove described invention for any two arbitrarily chosen frequency bandshaving a considerable separation between them, as long as the signalshave a real phase propagation constant at all cross-section of thestructure. However, in order that the excitation of spurious modes withhigh cross-polarization content be maintained at a low level, it isdesirable that the DDCT is conceived under such cross-sectionaldimensions between its two ends that propagation of these unwanted modesis not allowed as long as the near zero boundary susceptance conditionis not fulfilled in the particular frequency band under consideration.When this condition is applied in conjunction with the requirement forlow return loss characteristics, the principles of the present inventiongreatly facilitate in configuring a DDCT with efficient launchingcharacteristics; since, in this case it is possible to obtain goodreturn loss at two frequency bands even while one of the bandspropagates signals with very low phase propagation constant. A situationof this nature arises often in the design of the feed horn launchers foroperation in two bands with wide separation and where low levels ofspurious mode excitation must, also, be maintained.

We claim:
 1. In a transition device operable in a first frequency bandand a second, distinctly different frequency band comprising a waveguidehaving first and second ports and having a tapered interior boundarywall containing alternately positioned first and second type slots ofdistinct relative configuration aligned transverse to the axis of saidwaveguide, the improvement comprising: said first and second type slotseach configured near said first port to have (i) respective first andsecond susceptances for signals in said first frequency band, whichfirst and second susceptances are each non-zero and substantially equalin magnitude, with one of said first and second susceptances beingcapacitive and the other being inductive, and (ii) respective third andfourth susceptances for signals in said second, distinctly differentfrequency band, which third and fourth susceptances are each non-zeroand substantially equal in magnitude, with one of said third and fourthsusceptances being capacitive and the other being inductive, such thatsaid first and second susceptances, in combination, and said third andfourth susceptances, in combination, provide respective and simultaneoushigh susceptance mutual resonance conditions between adjacent ones ofsaid first and second type slots for said first and second frequencybands as are required for simultaneous matching of said device with acontinuous waveguide at said first port, for signals in said first andsecond frequency bands.
 2. A transition device of claim 1 wherein saidinterior boundary wall is circular.
 3. A transition device of claim 1wherein said first slots are deeper than said second slots.
 4. Atransition device of claim 2 wherein said first slots are deeper thansaid second slots.
 5. A transition device of claim 3 wherein saidinterior boundary wall is circular and has a smaller diameter at saidfirst port than at said second port.
 6. A transition device of claim 1,2, 3, 4 or 5 wherein said first type slots, near said first port, areconfigured to have said first susceptance capacitive for signals in saidfirst frequency band and to have said third susceptance inductive forsignals in said second frequency band, and said second type slots, nearsaid first port, are configured to have said second susceptanceinductive for signals in said first frequency band and to have fourthsusceptance capacitive for signals in said second frequency band.
 7. Atransition device of claim 6 wherein each of said first and second typeslots has an independent rate of change in their configurations,starting near said first port and continuing toward said second port, togradually suppress said mutual resonance conditions between adjacentslots and to achieve, at a first location in said waveguide remote fromsaid first port, a quarter wavelength self-resonance boundary conditionfor said first type slots for signals in said first frequency band and,at a second location in said waveguide remote from said first port, toachieve a quarter wavelength self-resonance boundary condition for saidsecond type slots for signals in said second frequency band to support,in said first and second slots respectively at said first and secondlocations, a balanced hybrid mode for signals in said respectivefrequency bands.
 8. A transition device of claim 7 wherein theconfiguration of said first type slots remains constant from said firstlocation of said waveguide to said second port and said configuration ofsaid second type slots remains constant from said second location ofsaid waveguide to said second port.
 9. A transition device of claim 8wherein said first and second slots become progressively less deep fromsaid first port to said first and second locations, respectively.